Metal parts which are exposed to high temperatures are often protected by specially-formulated coatings. As an example, turbine engine parts are often covered by thermal barrier coating (TBC) systems, which include a bond layer and a top layer (i.e., the TBC itself). Most TBC's are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. The stabilized zirconia is often applied as a powder in the form of hollow spheres, as described in U.S. Pat. No. 4,450,184 of Longo et al. For a jet engine, these protective coatings are applied to various surfaces, such as turbine engine blades and vanes, combustor liners, and combustor nozzles.
A variety of techniques are available for applying TBC systems. Examples include electron beam physical vapor deposition (EB-PVD), and plasma processes. Each technique has advantages which make it attractive for certain applications. Plasma-spray techniques are the methods of choice in some applications for several reasons. First, they do not usually require the expensive equipment employed in EB-PVD. Furthermore, plasma spray systems are very well suited for coating large parts, with maximum control over the thickness and uniformity of the coatings.
In most plasma spray techniques, an electric arc is typically used to ionize various gasses, such as nitrogen, argon, helium, or hydrogen, to form a plasma operating at temperatures of about 8000.degree. C. or greater. (When the process is carried out in an air environment, it is often referred to as air plasma or "AP".) The gasses are expelled from an annulus (or torch) at high velocity, creating a characteristic thermal plume. Powder material is fed into the plume--often in a direction substantially normal to the direction of the plume. The particles melt in the plasma and are accelerated toward the substrate being coated.
The quality of a coating applied by plasma spray is dependent on a number of factors. Some of these factors include spray distances (gun-to-substrate); selection of the number of spray-passes; powder feed rate, torch power, plasma gas selection; angle of deposition; pre-treatment of the substrate; post-treatment of the applied coating; and the like.
A TBC such as that based on yttria-stabilized zirconia sometimes fails because of the occurrence of cracks which run through the coating in a direction parallel to the substrate interface. The cracks follow interlaminar weaknesses in the coating structure. These sites of weakness often result from poor bonding between layers of deposited particles of molten ceramic powder (commonly referred to as "splats"). The poor bonding may occur because the substrate temperature is too low, so that one splat is quenched before it bonds to a previously-deposited splat. Sometimes, the poor bonding may result because the heat content of the particles reaching the substrate is too low. At other times, the powder particles have not sufficiently melted during their residence time in the plasma, resulting in weakly- bonded, granular material. The granular material (colloquially referred to as "garbage") can be deposited as continuous or discontinuous layers in the coating structure, and can become planes of weakness.
It is thought that th e characteristics of the coa ting particles themselves may have some effect on coating quality. The hollow spheres of stabilized zirconia are often made by the process described in the referenced Longo patent. In brief, the process begins with the production of spherical agglomerates of admixed powders, by way of a spray drying process. The agglomerated powders are held together by a water-soluble binder material such a polyvinyl alcohol. After sieving techniques have been used to reduce the particle size distribution, the spherical agglomerates are passed through a high-temperature, low velocity nitrogen/hydrogen plasma produced by a conventional plasma gun. The resulting powders are then air-quenched and collected as hollow spheres. It appears that a portion of a typical product mixture is made up of relatively thin-walled hollow spheres, while another portion of the mixture consists of an agglomeration of smaller, spherical agglomerates held together by additional powder.
From this discussion, it is apparent that the quality of a coating deposited by a plasma spray technique may be related to particular process parameters, as well as to the characteristics of the particles which are used to form the coating. Methods for depositing coatings of increased quality continue to be sought after in the relevant industries. These methods should involve the elimination or substantial reduction of delamination cracks in the coating structure, since those types of cracks reduce the service life of the coatings, i.e., they reduce the amount of time the protective coatings can be utilized before they must be replaced or extensively repaired.
Any now method should also be compatible with existing equipment, and with the other process steps involved in depositing protective coatings on metal-based substrates. Moreover, the resulting coatings should have performance characteristics which are equal or superior to those of the current art. This is especially true when the substrate is a high performance article like a turbine engine part.